Rotary dryers for drying of hydrous masses such as, for example, biological sludge have been proposed with the rotary dryer including an elongated rotatable drum having a drum casing, on an inner side of which are disposed a plurality of radially disposed vane elements which extend into an interior of the drum. The vane elements are arranged to lift the sludge when the drum is rotated, and at least some of the vane elements comprise a rake element including a number of ribs in a radial direction and bridge-like connecting elements, preferably, parallel to the axis of the drum. The connecting elements connect the free ends of the ribs to form openings.
Rotary dryers of the aforementioned type are used for drying many different types of hydrous masses such as, for example, biological sludge or comminuted masses of organic or biological materials, the water contents of which must be removed by drying.
In order to reduce the consumption of energy in the drying of the hydrous masses, a mechanical drying is often first carried out, with such mechanical drying being effected, for example, by the use of a press, prior to establishing a thermal drying. With mechanical drying of, for example, biological sludge, the water content can only be reduced to around 65-85%, so that there still is a very high content of water to be dried off.
By drying the sludge to a completely dry granulate, a stable and odorless biological inactive material is obtained which can be used as a fertilizer or soil improvement agent.
If, for example, the biological sludge is dried to a water content of less than 10%, preferably around 5%, the dried sludge is combustible with a calorific value of approximately 2500-3500 kcal/kg., and can thus be used as fuel, for example, in connection with the generation of thermal energy for the drying process.
U.S. Pat. No. 3,950,861 proposes a rotary dryer for drying of, for example, biological sludge, by a rotating drying drum which is equipped with lifting vanes which bring the sludge into better contact with the hot drying gas. In the configuration of the lifting vanes, special regard is paid to preventing the sludge from packing tightly on or around the vanes.
However, biological sludge from cleaning plants, which is more or less dried mechanically, has the very unfortunate characteristic that apparently a firm press-cake becomes deliquescent and sticky (tixotropic) when worked mechanically and will, for example, in a plant such as disclosed in U.S. Pat. No. 3,950,861, be kneaded into large clumps which are unstable for a rotational drying. In order to avoid this, material, which has already been dried, is mixed back in, so that from the start, the sludge is not tixotropic, but substantially fluid, otherwise one must introduce mechanical breaking elements which separate the large clumps to form smaller clumps in a manner described, for example, in U.S. Pat. No. 3,720,004 or DE-B-1,246,588.
In this connection, DE-B-1,246,588 proposes a rotating dryer for coal sludge. The drum has movable vane elements, with one side of the vane elements being hinged to an inner surface of the drum. The freely swinging ends of the vane elements will, in connection with violent strokes and loud noise, destroy the packed drying materials. The hinges are in the hot drying gas and will be destroyed by the heat and due to the wear from the drying materials.
The above noted known proposals have a number of disadvantages in that the mixing-back results in reduced capacity, increased energy consumption, complicated control, etc., and, in that the introduction of mechanical breaking elements results, among other things, in a complicated and expensive construction with high energy consumption and high maintenance costs.